This invention relates to thermally developable imaging materials such as photothermographic materials. More particularly, it relates to photothermographic imaging materials that exhibit decreased mottle and improved image uniformity upon exposure and development. The invention also relates to methods of imaging using these materials. This invention is directed to the photothermographic imaging industry.
Silver-containing photothermographic imaging materials that are developed with heat and without liquid development have been known in the art for many years. Such materials are used in a recording process wherein an image is formed by imagewise exposure of the photothermographic material to specific electromagnetic radiation (for example, visible, ultraviolet or infrared radiation) and developed by the use of thermal energy. These materials, also known as xe2x80x9cdry silverxe2x80x9d materials, generally comprise a support having coated thereon: (a) a photosensitive catalyst (such as silver halide) that upon such exposure provides a latent image in exposed grains that is capable of acting as a catalyst for the subsequent formation of a silver image in a development step, (b) a non-photosensitive source of reducible silver ions, (c) a reducing composition (usually including a developer) for the reducible silver ions, and (d) a hydrophilic or hydrophobic binder. The latent image is then developed by application of thermal energy.
In such materials, the photosensitive catalyst is generally a photographic type photosensitive silver halide that is considered to be in catalytic proximity to the non-photosensitive source of reducible silver ions. Catalytic proximity requires intimate physical association of these two components either prior to or during the thermal image development process so that when silver atoms, (Ag0)n, also known as silver specks, clusters, nuclei, or latent image, are generated by irradiation or light exposure of the photosensitive silver halide, those silver atoms are able to catalyze the reduction of the reducible silver ions within a catalytic sphere of influence around the silver atoms [D. H. Klosterboer, in Imaging Processes and Materials, (Neblette""s Eighth Edition), J. Sturge, V. Walworth, and A. Shepp, Eds., Van Nostrand-Reinhold, New York, 1989, Chapter 9, pp. 279-291]. It has long been understood that silver atoms act as a catalyst for the reduction of silver ions, and that the photosensitive silver halide can be placed in catalytic proximity with the non-photosensitive source of reducible silver ions in a number of different ways (see, for example, Research Disclosure, June 1978, item 17029). Other photosensitive materials, such as titanium dioxide, cadmium sulfide, and zinc oxide, have also been reported to be useful in place of silver halide as the photocatalyst in photothermographic materials [see for example, Shepard, J. Appl. Photog. Eng. 1982, 8(5), 210-212, Shigeo et al., Nippon Kagaku Kaishi, 1994, 11, 992-997, and FR 2,254,047 (Robillard)].
The photosensitive silver halide may be made xe2x80x9cin situxe2x80x9d, for example, by mixing an organic or inorganic halide-containing source with a source of reducible silver ions to achieve partial metathesis and thus causing the in situ formation of silver halide (AgX) grains throughout the silver source [see, for example, U.S. Pat. No. 3,457,075 (Morgan et al.)]. In addition, photosensitive silver halides and sources of reducible silver ions can be coprecipitated [see Usanov et al., J. Imag. Sci. Tech. 1996, 40, 104]. Alternatively, a portion of the reducible silver ions can be completely converted to silver halide, and that portion can be added back to the source of reducible silver ions (see Usanov et al., International Conference on Imaging Science, Sep. 7-11, 1998).
The silver halide may also be xe2x80x9cpreformedxe2x80x9d and prepared by an xe2x80x9cex situxe2x80x9d process whereby the silver halide (AgX) grains are prepared and grown separately. With this technique, one has the possibility of controlling the grain size, grain size distribution, dopant levels, and composition much more precisely, so that one can impart more specific properties to both the silver halide grains and the photothermographic material. The preformed silver halide grains may be introduced prior to, and be present during, the formation of the source of reducible silver ions. Co-precipitation of the silver halide and the source of reducible silver ions provides a more intimate mixture of the two materials [see for example, U.S. Pat. No. 3,839,049 (Simons)]. Alternatively, the preformed silver halide grains may be added to and physically mixed with the source of reducible silver ions.
The non-photosensitive source of reducible silver ions is a material that contains reducible silver ions. Typically, the preferred non-photosensitive source of reducible silver ions is a silver salt of a long chain aliphatic carboxylic acid having from 10 to 30 carbon atoms, or mixtures of such salts. Such acids are also known as xe2x80x9cfatty acidsxe2x80x9d or xe2x80x9cfatty carboxylic acidsxe2x80x9d. Silver salts of other organic acids or other organic compounds, such as silver imidazoles, silver tetrazoles, silver benzotriazoles, silver benzotetrazoles, silver benzothiazoles and silver acetylides have also been proposed. U.S. Pat. No. 4,260,677 (Winslow et al.) discloses the use of complexes of various inorganic or organic silver salts.
In photothermographic materials, exposure of the photographic silver halide to light produces small clusters containing silver atoms (Ag0)n. The imagewise distribution of these clusters, known in the art as a latent image, is generally not visible by ordinary means. Thus, the photosensitive material must be further developed to produce a visible image. This is accomplished by the reduction of silver ions that are in catalytic proximity to silver halide grains bearing the silver-containing clusters of the latent image. This produces a black-and-white image. The non-photosensitive silver source is catalytically reduced to form the visible black-and-white negative image while much of the silver halide, generally, remains as silver halide and is not reduced.
In photothermographic materials, the reducing agent for the reducible silver ions, often referred to as a xe2x80x9cdeveloperxe2x80x9d, may be any compound that, in the presence of the latent image, can reduce silver ion to metallic silver and is preferably of relatively low activity until it is heated to a temperature sufficient to cause the reaction. A wide variety of classes of compounds have been disclosed in the literature that function as developers for photothermographic materials. At elevated temperatures, the reducible silver ions are reduced by the reducing agent. In photothermographic materials, upon heating, this reaction occurs preferentially in the regions surrounding the latent image. This reaction produces a negative image of metallic silver having a color that ranges from yellow to deep black depending upon the presence of toning agents and other components in the imaging layer(s).
Differences Between Photothermography and Photography
The imaging arts have long recognized that the field of photothermography is clearly distinct from that of photography. Photothermographic materials differ significantly from conventional silver halide photographic materials that require processing with aqueous processing solutions.
As noted above, in photothermographic imaging materials, a visible image is created by heat as a result of the reaction of a developer incorporated within the material. Heating at 50xc2x0 C. or more is essential for this dry development. In contrast, conventional photographic imaging materials require processing in aqueous processing baths at more moderate temperatures (from 30xc2x0 C. to 50xc2x0 C.) to provide a visible image.
In photothermographic materials, only a small amount of silver halide is used to capture light and a non-photosensitive source of reducible silver ions (for example, a silver carboxylate) is used to generate the visible image using thermal development. Thus, the imaged photosensitive silver halide serves as a catalyst for the physical development process involving the non-photosensitive source of reducible silver ions and the incorporated reducing agent. In contrast, conventional wet-processed, black-and-white photographic materials use only one form of silver (that is, silver halide) that, upon chemical development, is itself converted into the silver image, or that upon physical development requires addition of an external silver source (or other reducible metal ions that form black images upon reduction to the corresponding metal). Thus, photothermographic materials require an amount of silver halide per unit area that is only a fraction of that used in conventional wet-processed photographic materials.
In photothermographic materials, all of the xe2x80x9cchemistryxe2x80x9d for imaging is incorporated within the material itself. For example, such materials include a developer (that is, a reducing agent for the reducible silver ions) while conventional photographic materials usually do not. Even in so-called xe2x80x9cinstant photographyxe2x80x9d, the developer chemistry is physically separated from the photosensitive silver halide until development is desired. The incorporation of the developer into photothermographic materials can lead to increased formation of various types of xe2x80x9cfogxe2x80x9d or other undesirable sensitometric side effects. Therefore, much effort has gone into the preparation and manufacture of photothermographic materials to minimize these problems during the preparation of the photothermographic emulsion as well as during coating, use, storage, and post-processing handling.
Moreover, in photothermographic materials, the unexposed silver halide generally remains intact after development and the material must be stabilized against further imaging and development. In contrast, silver halide is removed from conventional photographic materials after solution development to prevent further imaging (that is, in the aqueous fixing step).
In photothermographic materials, the binder is capable of wide variation and a number of binders (both hydrophilic and hydrophobic) are useful. In contrast, conventional photographic materials are limited almost exclusively to hydrophilic colloidal binders such as gelatin.
Because photothermographic materials require dry thermal processing, they present distinctly different problems and require different materials in manufacture and use, compared to conventional, wet-processed silver halide photographic materials. Additives that have one effect in conventional silver halide photographic materials may behave quite differently when incorporated in photothermographic materials where the underlying chemistry is significantly more complex. The incorporation of such additives as, for example, stabilizers, antifoggants, speed enhancers, supersensitizers, and spectral and chemical sensitizers in conventional photographic materials is not predictive of whether such additives will prove beneficial or detrimental in photothermographic materials. For example, it is not uncommon for a photographic antifoggant useful in conventional photographic materials to cause various types of fog when incorporated into photothermographic materials, or for supersensitizers that are effective in photographic materials to be inactive in photothermographic materials.
These and other distinctions between photothermographic and photographic materials are described in Imaging Processes and Materials (Neblette""s Eighth Edition), noted above, Unconventional Imaging Processes, E. Brinckman et al., Eds., The Focal Press, London and New York, 1978, pp. 74-75, in Zou et al., J. Imaging Sci. Technol. 1996, 40, pp. 94-103, and in M. R. V Sahyun, J. Imaging Sci. Technol., 1998, 42, 23.
Problem to be Solved
Thermally developable materials have gained widespread use in several industries, particularly in radiography. Such materials are usually constructed by coating layer formulations from solution and removing as much of the solvent as possible by drying. Problems that arise with this manufacturing process include the formation of coating defects that can be attributed to the various coating and drying conditions and procedures.
One such coating defect, referred to as xe2x80x9cmottlexe2x80x9d, arises from an unevenness in the distribution of solid materials formed within a coating as solvent is removed during drying [see, for example, Modern Coating and Drying Technology, Eds. E. D. Cohen and E. B. Gutoff, Eds., VCH Publishers, New York, 1992, p. 288]. It is believed to be caused by a non-uniform airflow blowing the coating around in the early stages of the drying process when the coating is still quite fluid. This can occur in the coating before it enters the dryer, as it enters the dryer, or in the dryer and can be more severe with coating solvents of increased volatility [see, for example, Coating and Drying Defects: Troubleshooting Operating Problems, E. B. Gutoff and E. D. Cohen, John Wiley and Sons, New York, 1995, p. 203].
In a coated material, mottle appears as an irregular pattern of non-uniform density that appears blotchy when viewed. The pattern may take on an orientation or direction. The scale can be quite small or quite large and may be on the order of centimeters. The blotches may appear to have different colors or shades of colors and can be gross or subtle.
Mottle may not be readily apparent in undeveloped photothermographic materials but upon thermal development it becomes more evident. For example, in black-and-white photothermographic materials upon development the resulting non-uniform image density may appear as shades of gray.
Various techniques have been used for reducing mottle in coated materials. For example, to reduce the severity of non-uniform airflow on the undried coating, dryer airflow and web speed can be reduced. However, this can lower the coating line speed, reduce manufacturing efficiency, and increase manufacturing costs.
Careful control of oven design, as well as coating and/or drying conditions, have also been used to control mottle. Some of these techniques are described in U.S. Pat. No. 4,051,278 (Democh), U.S. Pat. No. 5,881,476 (Strobush et al.), and U.S. Pat. No. 5,621,983 (Ludemann et al.).
Surfactants have also been incorporated into coating formulations used to reduce mottle, including for example fluorinated surfactants as described for example in U.S. Pat. No. 5,380,644 (Yonkoski et al.) and U.S. Pat. No. 5,532,121 (Yonkoski et al.). However, the use of surfactants may lead to other problems as they may adversely affect the sensitometric properties of the imaging materials as well as their ability to be fed and transported within the imaging apparatus.
The techniques described above may limit the manufacturability of the materials, produce other undesirable properties in the materials, or may not sufficiently reduce mottle for all imaging material requirements.
Furthermore, it is known in the imaging arts, including photothermographic art, to incorporate acutance dyes into imaging layers to improve sharpness [see for example, U.S. Pat. No. 5,380,635 (Gomez et al.) and U.S. Pat. No. 5,922,529 (Tsuzuki et al.)]. It is also known to add such materials to reduce interference fringes during laser exposure [see for example, U.S. Pat. No. 5,998,126 (Toya et al.)], and to reduce xe2x80x9cwoodgrainxe2x80x9d [see for example, EP 0 792 476 B1 (Geisler et al.)]. The acutance dyes are incorporated into the photothermographic materials due in an amount necessary to provide an absorbance in the range of 0.05 to 0.6. Higher absorbance is not believed to provide additional benefits in image sharpness or reduction of interference fringes.
It is desirable to reduce the formation of mottle during manufacture of photothermographic materials without the use of surfactants or modification of coating and drying procedures. There is also a need to accomplish this without an unacceptable loss in sensitivity.
This invention provides a photothermographic material that comprises a support having thereon one or more thermally-developable imaging layers comprising a binder and in reactive association, a photosensitive silver halide, a non-photosensitive source of reducible silver ions, and a reducing composition for the non-photosensitive source of reducible silver ions, wherein the one or more thermally-developable imaging layers further comprise one or more radiation absorbing substances that provide a total absorbance in the one or more thermally-developable imaging layers of greater than 0.6 and up to and including 3 at an exposure wavelength, the one or more radiation absorbing substances being chosen such that when they are incorporated into the one or more thermally-developable layers to provide a total absorbance of from about 0.6 to about 1.2, the log of the exposure (in ergs/cm2) required to produce an image density of 1+Dmin, increases by less than 0.4 per absorbance unit.
The photothermographic materials of this invention exhibit reduced mottle after imaging and thermal development without an unacceptable loss in sensitivity. The appearance of mottle is reduced without having to use surfactants in the coated layers and without adjusting coating and drying conditions in manufacturing operations, thereby providing an improved imaging material with good manufacturability.
These advantages have been achieved by incorporating certain radiation absorbing compounds (generally dyes) in the one or more thermally developable imaging layers of the photothermographic materials in a quantity sufficient to provide a total optical density (or absorbance) in those layers of greater than 0.6 and up to and including 3. These absorbing compounds may not reduce the susceptibility of the wet coatings to being blown about by non-uniform airflow, but they reduce the appearance of mottle in the imaged and developed photothermographic material.
To achieve optimal sensitivity, not just any dyes that provide the absorbance greater than 0.6 can be used in the practice of this invention. The most useful radiation absorbing compounds must provide the desired high absorbance without requiring too great of an increase in imaging exposure to obtain desired image density. This is achieved by requiring the radiation absorbing compounds meet the additional parameter that the log of the exposure (in ergs/cm) required to produce a density of 1+Dmin should increase by less than 0.4 per absorbance unit when the radiation absorbing compound is present to provide an absorbance between 0.6 and 1.2.
In addition, the dyes must not interfere with the manufacturing process and permit high speed coating and drying of the photothermographic material.
When comparing radiation absorbing compounds, sensitivity loss should be determined at an absorbance between 0.6 and 1.2. This feature does not limit the use of the invention to this absorbance range, however, but enables a more even comparison among compounds to determine which ones are preferable or optimal for use in a given photothermographic material. The best radiation absorbing compounds can then be used to provide an absorbance of 2.0 or more if contrast is properly adjusted to maintain desired sensitometric properties.
In another embodiment, this invention provides a photothermographic material having one or more thermally developable imaging layers on both sides of the support.
Further, a method of this invention for forming a visible image comprises:
A) imagewise exposing the black and white photothermographic material described above to electromagnetic radiation at a wavelength greater than 700 nm to form a latent image, and
B) simultaneously or sequentially, heating the exposed photothermographic material to develop the latent image into a visible image.
When the photothermographic materials of this invention are heat-developed, as described below, in a substantially water-free condition after, or simultaneously with, imagewise exposure, a silver image (preferably a black-and-white silver image) is obtained. The photothermographic material may be exposed in step A using an laser, a laser diode, a light-emitting screen, CRT tube, a light-emitting diode, a light bar, or other radiation source readily apparent to one skilled in the art.
In some embodiments of the imaging method of this invention, the photothermographic material has a transparent support and the imaging method further includes:
C) positioning the exposed and heat-developed photothermographic material between a source of imaging radiation and an imageable material that is sensitive to the imaging radiation, and
D) thereafter exposing the imageable material to the imaging radiation through the visible image in the exposed and heat-developed photothermographic material to provide an image in the imageable material.
Preferred embodiments of this invention include black-and-white photothermographic materials each comprising a support having on one side thereof:
a) a thermally-developable imaging layer comprising a hydrophobic binder and in reactive association, a photosensitive silver bromide or silver bromoiodide, or mixtures thereof, one or more non-photosensitive silver carboxylates, at least one of which is silver behenate, and a merocyanine or cyanine spectral sensitizing dye,
b) a protective layer that is farther from the support than the imaging layer,
the photothermographic material also comprising an antihalation layer on the backside of the support, the antihalation layer comprising a binder and at least one antihalation dye,
wherein the thermally-developable imaging layer further comprises one or more radiation absorbing substances that provide a total absorbance in the one or more thermally-developable imaging layers greater than 0.6 and up to and including 3 at an exposure wavelength,
the one or more radiation absorbing substances chosen such that when they are incorporated into the thermally-developable layer to provide a total absorbance of from about 0.6 to about 1.2, the log of the exposure (in ergs/cm2) required to produce an image density of 1+Dmin increases by less than 0.4 per absorbance unit, and the one or more radiation absorbing substances being cyanine, hemicyanine, merocyanine, squaraine, or oxanol dyes, or mixtures thereof.